Power take off clutch engagement control system

ABSTRACT

A power take off (PTO) clutch control system is disclosed herein. The PTO clutch is a hydraulic clutch which is operated by a proportional valve capable of pressurizing the clutch with hydraulic fluid at a pressure related to the pulse width of a pulse width modulated (PWM) signal applied to the valve. The PWM signal is produced by a controller which monitors the output shaft speed and movement to determine the time at which the shaft begins movement. Prior to shaft movement, the PWM signals control the proportional valve to engage the clutch slowly and smoothly to avoid producing unacceptably high torques in the PTO shaft connected to the clutch during clutch engagement.

FIELD OF THE INVENTION

The present invention relates to the power take off (PTO) for anagricultural vehicle such as a tractor. In particular, the presentinvention relates to a control system for controlling the operation of aPTO clutch to provide relatively smooth clutch engagement.

BACKGROUND OF THE INVENTION

PTOs are used on agricultural vehicles such as tractors to provide powerfor equipment or implements such as combines, mowers and spreaders. Asthe use of PTOs developed, most tractor manufactures standardized on1000 RPM and 540 RPM PTOs. This standardization involved the use of acommon size shaft and spline arrangement for each RPM rating. When theshaft sizes were standardized years ago, tractors had relatively lowhorsepower (e.g. 30 to 50 horsepower). Accordingly, the torque output ofa PTO shaft was limited by the horsepower of the tractor.

Modern tractors commonly have horsepower ratings in excess of 100horsepower. However, the shaft sizes for PTOs have not changed due tothe need to maintain compatibility with older equipment and maintain thestandardization for PTOs. Thus, the torque output of PTOs for manymodern tractors is no longer limited by the tractor horsepower. Rather,the torque output is limited by the strength of the PTO shaft and thefailure thereof. For the very high horsepower tractors (e.g. over 130horsepower) manufacturers have eliminated the 540 RPM PTO shaft. Due tothe gear reduction required to achieve a PTO speed of 540 RPM at engineidle, the very high horsepower tractors can apply a torque to the 540RPM shaft in excess of that required for the shaft to fail.

In addition to causing PTO shaft failures, the torque produced by thehigher horsepower tractors can accelerate equipment attached to therespective PTO at a rate which can damage the equipment. In an attemptto limit acceleration of the PTO shaft, PTO clutch controls have beenused to monitor the speed of the input and output shafts of the PTOclutch. Based upon the monitored speeds, the controls turn the clutchcontrol valve ON and OFF in a cyclic fashion to limit the rate at whichthe PTO shaft is accelerated. However, since PTO clutch control valvesare typically either fully ON or OFF, the acceleration of the PTO shaftoccurs in a step-wise manner which may in itself introduce undesirabletorque pulses into the PTO shaft and associated implement.

In view of the problems involved in the control of PTO shafts in highhorsepower tractors, it would be useful to provide a PTO clutch controlsystem for protecting PTO shafts from catastrophic failure, andproviding PTO shaft accelerations at rates which protect the shafts andattached implements during clutch engagement.

SUMMARY OF THE INVENTION

The present invention relates to a PTO control system for vehicles suchas farm tractors including a power take-off (PTO) shaft for supplyingrotational motion to an implement of the type which may be stationary ortowed by the tractor. Power is transferred to the PTO shaft by a clutchincluding an input shaft coupled to a power source and an output shaftcoupled to the PTO shaft, wherein the clutch includes a plurality ofclutch plates operable to translate through a distance prior to clutchengagement. Upon clutch plate engagement, the clutch transmits torquebetween the input and output shafts to reduce the peak torques whichnormally occur during clutch engagement.

The control system includes a clutch control configured to translate theclutch plates through the distance to engage the clutch plates inresponse to a first control signal, and an output transducer disposed togenerate an output signal representative of the rotational speed of theoutput shaft. A system control circuit is coupled to the clutch control,and the output transducer. The control circuit monitors the outputsignal and applies the first control signal to the clutch control untilthe output signal is representative of a rotational speed of the outputshaft greater than zero.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram of a PTO drive and control system;

FIG. 2 is a schematic block diagram representative of the circuitconfiguration for the controller of the control system;

FIGS. 3A and 3B are flow charts representative of the control functionof the control system;

FIG. 4 is a graphical representation of a control signal applied to thehydraulic valve of the control system; and

FIG. 5 is a graphical representation of the actual and desiredaccelerations of the PTO shaft.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Turning to FIG. 1, a power takeoff (PTO) clutch and brake control system10 for an agricultural vehicle such as a tractor schematicallyrepresented by the dashed line labeled 12 is shown. With the exceptionof the PTO clutch control system 10, tractor 12 may be a conventionalagricultural tractor of the type including an engine 14 havingconventional accessories such as an alternator 16. Engine 14 is thepower source for tractor 12 and, in addition to providing power to thedrive wheels (not shown) of tractor 12, provides the power to applyrotational motion to a multi-plate hydraulically actuated PTO clutch 18.

Control system 10 includes a controller 20 (e.g. a digitalmicroprocessor such as the Intel TN83C51FA), a PTO on/off switch 22, aPTO input clutch speed transducer 24, and an output clutch speedtransducer 26, a PTO status switch 27, a normally closed, solenoidoperated, hydraulic, proportional clutch control valve 28. By way ofexample, transducers 24 and 26 may be variable reluctance sensors;however, signals representative of the rotational speed of the inputshaft of clutch 18 may be derived from alternator 16.

In addition to controlling clutch 18, system 10 may control a hydraulicbrake 30 which inhibits rotational motion of PTO output shaft 32 whenclutch 18 is not fully engaged. System 10 includes a hydraulic valve 34connected to brake 30 by hydraulic conduit 38. Valve 34 engages anddisengages brake 30. Brake 30 is biased to inhibit rotation of shaft 32.Accordingly, valve 34 is normally closed, and opened when brake 30 is tobe released. Depending upon the application and the configuration ofvalve 28 and the hydraulic conduit 36 which connects valve 28 to clutch18, valve 34 may be eliminated by connecting brake 30 directly toconduit 36. Accordingly, as valve 28 applies pressurized hydraulic fluidto engage clutch 18, the pressurized fluid would also release brake 30.By configuring conduits 36 and 38 appropriately, the engagement ofclutch 18 and releasing of brake 30 can be synchronized to avoidengaging clutch 18 without appropriately releasing brake 30.

Transducers 24 and 26 are coupled to digital inputs of controller 20 byelectrical conductors 25 and 29, and conditioning circuits 38 which maybe integral to controller 20. Conditioning circuits 38 filter radio andother undesirable frequencies of interference from the signals producedby transducers 24 and 26 or alternator 16, and introduced in conductors25 and 29. Additionally, circuit 38 places the signals produced bytransducers 24 and 26 or alternator 16 within a 5 V range and providesthese signals with a generally squarewave configuration which can beappropriately sampled by controller 20. In operation, transducer 24produces a signal representative of (e.g. proportional to) therotational speed of the input shaft 19 of clutch 18, and transducer 26produces a signal representative of (e.g. proportional to) therotational speed of the clutch output shaft 32. Accordingly, the signalsapplied to controller 20 by transducers 24 and 26 have a generallysquarewave configuration with a frequency proportional to the rotationalspeed of input shaft 19 and output shaft 32, respectively.

Switches 22 and 27 each include an associated conditioning circuit 40and 42, respectively which may be integral to controller 20. Dependingupon the application, circuits 40 and 42 may provide signal inversionand appropriate filtering to eliminate switch bounce. However, dependingupon the type of controller 20 used, circuits 40 and 42 may beeliminated. The signals produced by switches 22 and 27 are applied todigital inputs of controller 20 via electrical conductors 23 and 31,respectively.

Hydraulic valves 28 and 34 are coupled to digital outputs of controller20 by appropriate amplification and signal conditioning circuits 44 and46 integral to controller 20, and electrical conductors 48 and 50,respectively. As will be discussed in detail below, controller 20applies a pulse-width modulated (PWM) signal to valve 28 via electricalconductor 48 and circuit 44, and applies a digital on/off signal tovalve 34 via electrical conductor 50 and circuit 46. Due to the natureof the solenoids which operate valves 28 and 34, amplification andisolation circuits 44 and 46 are required to produce a control signalhaving sufficient voltage and current to operate valves 28 and 34.Additionally, due to inductive kickbacks which may potentially beproduced by the solenoids of valves 28 and 34, isolation may be requiredin circuits 44 and 46 to protect controller 20.

Turning to the operation of valve 28, valve 28 is a proportionalhydraulic valve which applies hydraulic fluid to clutch 18 from thesystem hydraulic fluid source 52 at a pressure which is related to (e.g.proportional to) the time-averaged voltage applied to the solenoidassociated with valve 28. Thus, the pressure of the fluid applied toclutch 18 via hydraulic conduit 36 by valve 28 may be controlled byapplying a variable voltage signal to valve 28, or may be controlled byapplying a PWM signal to the solenoid of valve 28. Where a PWM signal isapplied to the solenoid of valve 28 to control the pressure of thehydraulic fluid applied to clutch 18, as in the presently preferredembodiment, the pressure of the fluid is proportional to the pulse widthof the PWM signal produced by controller 20.

As discussed above, clutch 18 is a multi-plate hydraulic clutch. Thistype of clutch is capable of transferring a torque from clutch inputshaft 19 to output shaft 32, where the torque is generally proportionalto the pressure of the hydraulic fluid applied to clutch 18. (Shaft 19is coupled to engine 14. Shaft 32 is directly coupled to the 1000 RPMPTO (high speed PTO) output shaft 33 (or a high speed output shaft ofanother speed rating such as 750 RPM), and is coupled to the 540 RPM PTO(low speed PTO) shaft 35 by a reduction gear 37.) Accordingly, thetorque transferred between shafts 19 and 32 will be generallyproportional to the pulse width (duty cycle) of the PWM signals appliedfrom controller 20 to the solenoid of valve 28. Ideally, it may beconvenient to have the torque transferred between shafts 19 and 32exactly proportional to the pulse width of the PWM signals applied tothe solenoid of valve 28; however, in mechanical systems, such arelationship is difficult to obtain. Accordingly, controller 20 isprogrammed to compensate for the inability to obtain suchproportionality, and overall non-linearity in the electronics andmechanism of the control system 10.

Referring to FIG. 2, controller 20 includes a memory circuit 54 havingRAM and ROM, and is configured (programmed) to provide the operations ofa speed sensing circuit 56, a timing circuit 58, a switch statusmonitoring circuit 60, a signal processing circuit 62, and a valvecontrol signal output circuit 64. The direction and channels for dataflow between circuits 54, 56, 58, 60, 62 and 64 are shown in FIG. 2. TheROM of memory circuit 54 stores those values required for system 10initialization, and the constants required for the operation of certainprograms run by controller 20. The RAM of memory 54 provides thetemporary digital storage required for controller 20 to execute thesystem program.

Speed sensing circuit 56 receives the signals from transducers 24 and 26which are applied to conductors 25 and 29, and converts the signals todigital values representative of the rotational speeds of shafts 19 and32, respectively. Timing circuit 58 includes counters which are utilizedby signal processing circuit 62 while executing the programmingrepresented by the flow charts of FIGS. 3A and 3B. Switch statusmonitoring circuit 60 converts the signals applied by switches 22 and 27to conductors 23 and 31 to digital values representative of the statusof these switches. Valve control signal output circuit 64 produces a 400Hz PWM signal applied to the solenoid of valve 28 via conductor 48 andisolation circuit 44 having an appropriate pulse width, and produces theon/off signal applied to valve 34 via conductor 50 and circuit 46. Asbriefly discussed below, the program executed by controller 20 isexecuted at 100 Hz. Thus, the pulse width of the signal produced bycircuit 64 is updated every 10 milliseconds or every 4 cycles of the PWMsignal.

The operation of signal processing circuit 62 will now be described indetail in reference to FIGS. 3A, 3B, 4 and 5. (FIGS. 3A and 3B representthe operational steps of the program run by controller 20.) Upon startup(step 66), controller 20 reads the ROM of memory circuit 54 andinitializes the counter in timing circuit 58 to a number of countsrepresentative of 6 seconds. In addition, controller 20 initializesthose other variables and constants which may be utilized in theprogramming of controller 20 (step 68). In step 70, circuit 62 reads thedigital value representative of the status of PTO switch 22 from circuit60, and returns if switch 22 has not been closed. If switch 22 isclosed, after it was detected open, circuit 60 executes the stepsrequired to begin engagement of clutch 18.

In step 71, circuit 62 accesses circuit 60 to determine if switch 22 wasopened and closed. If switch 22 was opened and closed, circuit 62 setsthe counts in timing circuit 58 to a number representative ofapproximately 2 seconds (step 73). If switch 22 was not opened andclosed, circuit 62 advances to step 72.

In step 72, circuit 62 reads the digital value representative of thestatus of switch 27 from circuit 60 and determines whether or not thePTO is operating as a 1000 RPM PTO or a 540 RPM PTO. If switch 27produces a signal representative of a 540 RPM PTO (low speed), a LOW PTOflag is set. In step 74, circuit 62 determines whether or not the LOWPTO flag is set. If the LOW PTO flag is set, circuit 62 calculates thetorque limit for clutch 18 at step 75 and stores a value in the RAM ofcircuit 54 representative of the maximum pulse width of the PWM signalto be applied to the solenoid of valve 28 during operation of the 540RPM PTO. The maximum pulse width depends upon the configuration oftractor 12, and is set so that the torque transferred by clutch 18 isless than the maximum torque at which the 540 RPM PTO shaft will fail.

Since the reduction required to reduce the speed of the 540 RPM shaft toapproximately 50% of the 1000 RPM shaft is approximately 2 to 1, atorque is applied to the 540RPM shaft which is approximately twice aslarge as the torque which can be applied to the 1000 RPM shaft for givenengine torque. Accordingly, the maximum pressure applied to the clutchthrough the valve 28 during operation of the 540 RPM shaft to transmitthe same torque is approximately 50% of the maximum pressure applied tothe clutch through the valve 28 during the operation of the 1000 RPM PTOshaft. This pressure is controlled by changing the width of the PWMsignal applied. The maximum pulse width value of the PWM signalassociated with the 540 RPM PTO shaft is stored in the ROM of circuit54. At step 74, if circuit 62 determines that the LOW PTO flag is notset, circuit 62 will utilize the maximum pulse width value stored incircuit 54 which is associated with the maximum torque clutch 18 cantransfer between shafts 19 and 35 during operation of the 540 RPM PTOshaft, without causing failure of the 540 RPM PTO shaft due to torqueoverload.

In step 76, circuit 62 reads the digital values representative of therotational speeds of input shaft 19 and output shaft 32 from circuit 56.In step 78, circuit 62 compares the speeds of shafts 19 and 32. If theshaft speeds are the same, circuit 62 resets timing circuit 58 to acount representative of 2 seconds, and sets a STEADY STATE flag (step80). Subsequently, circuit 62 loops to execute step 102 and the stepsbeginning at step 100. At step 102 the pulse width value is increased by1.00%. If the shaft speeds are different, processing continues at step82.

In step 82, circuit 62 determines whether or not the STEADY STATE flagis set. If the STEADY STATE flag is set, circuit 62 determined if thespeed difference between shafts 19 and 22 is greater then five percent(5%) (step 83). If it is greater than five percent, the time counter isdecremented by 2.5 milliseconds (step 84), and circuit 62 jumps to theprogramming associated with steps 102 and then 100. If the STEADY STATEflag is not set, circuit 62 goes to step 86 wherein circuit 62decrements the counter of circuit 58 by counts representative of 2.5milliseconds. (The programming represents by the flow charts of FIGS. 3Aand 3B runs at a rate of approximately 100 Hz. Accordingly, to decrementthe timer counter in circuit 58, the counter must be decremented by thenumber of counts associated with 10 milliseconds.)

In step 88, circuit 62 reads the value representative of the rotationalspeed of output shaft 32 to determine whether or not shaft 32 is moving.If shaft 32 is moving, circuit 62 applies a digital signal to circuit64, where circuit 64 responds to the signal by applying a signal toconductor 50 which causes valve 34 to release brake 30 (step 92). Atstep 90, if shaft 32 is not moving, circuit 62 reads the time from timercircuit 58 associated with the times since the PTO switch was closed andsets the pulse width value to a predetermined percentage (e.g. 20%) ofthe maximum pulse width value either set at step 75 in the case ofoperation at 540 RPM, or read from circuit 54 in the case of operationat 1000 RPM, if switch 22 has been closed for 300 milliseconds or less.If the time is greater than 300 milliseconds, the pulse width value isincreased by 0.1% for each 10 millisecond increment of time elapsedsubsequent to switch 22 being closed for 300 milliseconds. After settingthe pulse width value at step 90, circuit 62 jumps to step 104.

In general, steps 88 and 90 are provided to produce smooth engagement ofclutch 18. More specifically, before the plates of clutch 18 engage, acertain volume of hydraulic fluid must be provided to clutch 18 beforethe clutch plates of clutch 18 travel through the distance required toengage the clutch plates. During this clutch filling process, it isundesirable to apply hydraulic fluid to the clutch at a fixed orundesirably high pressure since the clutch will abruptly apply torquefrom shaft 19 to shaft 32. Such an abrupt application of torque canpotentially cause damage to shaft 32 or an associated implementconnected to the PTO output shaft. By initiating the filling of clutch18 with a pressure equivalent to the pre-stress force applied by theclutch springs, the clutch plates move relatively slowly towardengagement, and the pressure is increased gradually until engagement.This process prevents the abrupt transfer of torque from shaft 19 toshaft 32.

As shown in FIG. 4, the pulse width of the PWM signal is plotted againsttime. As shown, the first motion of the output shaft occurs at time T1(i.e., initial clutch engagement), the pulse width of the PWM signalhaving been initiated at a certain % duty cycle (e.g. 20%) at time T0and increased in gradual steps until output shaft 32 begins moving asdetermined at step 88. At time T2, the clutch is fully locked up.

Referring to FIG. 3B, in step 94, circuit 62 calculates a desiredacceleration by dividing the speed at shaft 19 by 2.5 seconds. Ingeneral, step 94 is the start of the process for controlling clutch 18to accelerate output shaft 32 relative to shaft 19 until the speed ofshaft 32 reaches its steady state speed (no clutch 18 slip) which equalsor is proportional to the speed of shaft 19. The acceleration of shaft94 is calculated based upon 2.5 seconds, which was selected based uponexperimentation to provide optimum acceleration of shaft 32. However,depending upon the system configuration, this time period may be variedaccording to the particular tractor and PTO application. The calculatedacceleration serves as a reference for accelerating shaft 32 relative toshaft 19.

In step 96, circuit 62 calculates the shaft acceleration by reading thecurrent speed of shaft 32 from circuit 56, and the speed of shaft 32monitored during the previous loop through steps 70-108. Steps 70-108are executed every 10 milliseconds; thus, the shaft acceleration is thechange in shaft speed between program loops divided by 10 milliseconds.If the actual acceleration of shaft 32 is less than the desired shaftacceleration, the current pulse width is increased by 0.1% (step 98). Ifthe actual acceleration of shaft 32 is greater than or equal to thedesired acceleration, the pulse width value is not changed. In certainsystems, it may be desirable to reduce the pulse width value when theactual acceleration of shaft 32 is greater than the desiredacceleration. However, this type of control may cause hunting, and thus,an acceleration of shaft 32 which is not smooth. Accordingly, in thepresently preferred embodiment of system 10, the pulse width value isnot modified when the actual acceleration of shaft 32 exceeds thedesired acceleration.

Referring to FIG. 4, the increase in the pulse width value which occursduring the execution of steps 94, 96 and 98 is shown between times T1and T2. As shown, this pulse width value is increased incrementally at arate of 0.1% change at each 10 millisecond interval, if the actualacceleration is less than the desired acceleration. Referring to FIG. 5,examples of the desired and actual speeds for shaft 32, and engine speedare plotted against time. As shown, at time T1, shaft 32 begins torotate, and at time T2, the speed of shaft 32 equals the speed of shaft19 (lock-up). At time T2, the speeds of shafts 19 and 32 are equal orproportional, and circuit 62 executes steps 100, 101 and 102 to ramp upthe pulse width value to produce a clutch pressure in clutch 18associated with the maximum torque to be transmitted between shafts 32and 19. In step 100, the current pulse width value is compared with themaximum pulse width value set determined at step 75 in case of operationat 540 RPM, and the PWM value stored in circuit 54 in case of operationat 1000 RPM. If the current pulse width value set at step 98 or step 102is greater than the maximum pulse width value, the pulse width value isset to the maximum pulse width value (step 101).

In step 104, circuit 62 checks the count of the timer in circuit 58 todetermine whether or not the timer has timed out. If the timer equals 0,then either motion of shaft 32 did not occur within 6 seconds (timercount at initialization), or the speed difference between shafts 19 and32 subsequent to time T2 (lock-up) has been greater than 5% for morethan 2 seconds which indicates undesirable slippage in clutch 18. Instep 104, circuit 62 also determines if the speed of shaft 19 has gonebelow 650 RPM. If either the timer count has reached 0 or the speed ofshaft 19 has gone below 650 RPM, circuit 62 sets the pulse width to zero(step 105). In step 106, circuit 62 applies the present pulse widthvalue to circuit 64. In response, circuit 64 applies a pulse widthmodulated signal to valve 28 via conductor 48 at a frequency of 400 Hzwith a pulse width corresponding to the current pulse width value whichwill be updated upon the next execution of steps 70 through 106. In step108, circuit 62 returns to the execution of step 70.

Although various features of the control system are described andillustrated in the drawings, the present invention is not necessarilylimited to these features and may encompass other features disclosedboth individually and in various combinations. For example, developmentsin PTO clutches may make electric clutches cost effective for PTOapplications. Accordingly, hydraulic clutch 18 and control valve 28 maypotentially be replaced with an associated electric clutch and electricclutch control circuit.

What is claimed is:
 1. In a vehicle having a power source for producingrotational motion, a power take-off (PTO) shaft for supplying rotationalmotion to at least one piece of equipment other than the vehicle, and aclutch including an input shaft coupled to the power source and anoutput shaft coupled to the PTO shaft, wherein the clutch includes aplurality of clutch plates operable to translate through a distanceprior to clutch engagement and, upon clutch plate engagement,transmitting torque between the input and output shafts, a controlsystem comprising:a clutch control configured to translate the clutchplates through the distance to engage the clutch plates in response to afirst control signal; an output transducer disposed to generate anoutput signal representative of the rotational speed of the outputshaft; and a control circuit coupled to the clutch control, and theoutput transducer, and being configured to monitor the output signal andapply the first control signal to the clutch control until the outputsignal is representative of a rotational speed of the output shaftgreater than zero.
 2. The system of claim 1, further comprising a brakecoupled to the clutch control and disposed to inhibit rotation of theoutput shaft when the clutch is disengaged and when the first controlsignal is applied to the clutch control.
 3. The system of claim 1,further comprising:a brake control coupled to the control circuit; and abrake coupled to the brake control, the brake being disposed to inhibitrotation of the output shaft when the clutch is disengaged and when thefirst control signal is applied to the clutch control.
 4. The system ofclaim 1, wherein the torque transmitted by the clutch between the inputand output shafts is dependent upon the pressure between the clutchplates and the clutch control is further configured to force the clutchplates together at a pressure defined by second control signals appliedto the clutch control, the system further comprising an input transducerdisposed to generate an input signal representative of the rotationalspeed of the input shaft, the input transducer being coupled to thecontrol circuit and the control circuit being further configured tomonitor the input signal and, upon monitoring an output shaft speedgreater than zero, applying second control signals to the clutch controlto increase the pressure between the clutch plates at a pressurizationrate which transmits torque from the input to the output shaft at levelswhich increase the rotational velocity of the output shaft at a rateless than a predetermined rate.
 5. In a vehicle having a power sourcefor producing rotational motion, a power take-off (PTO) shaft forsupplying rotational motion to at least one piece of equipment otherthan the vehicle, and a clutch including an input shaft coupled to thepower source and an output shaft coupled to the PTO shaft, wherein theclutch includes a plurality of clutch plates operable to translatethrough a distance prior to clutch engagement and, upon clutchengagement, transmitted torque between the input and output shafts, acontrol system comprising:a clutch control configured to translate theclutch plates through the distance to engage the clutch plates inresponse to a first control signal; an output transducer disposed togenerate an output signal representative of the rotational speed of theoutput shaft; and a control circuit coupled to the clutch control, andthe output transducer, and being configured to monitor the output signaland apply the first control signal to the clutch control until theoutput signal is representative of a rotational speed of the outputshaft greater than zero, wherein the torque transmitted by the clutchbetween the input and output shafts is dependent upon the pressurebetween the clutch plates and the clutch control is further configuredto force the clutch plates together at pressure defined by secondcontrol signals applied to the clutch control, the system furthercomprising an input transducer disposed to generate an input signalrepresentative of the rotational speed of the input shaft, the inputtransducer being coupled to the control circuit and the control circuitbeing further configured to monitor the input signal and, uponmonitoring an output shaft speed greater than zero, applying secondcontrol signals to the clutch control to increase the pressure betweenthe clutch plates at a pressurization rate which transmits torque fromthe input to the output shaft at levels which increase the rotationalvelocity of the output shaft at a rate less than a predetermined rate,and wherein the PTO shaft will fail at a predetermined maximum torqueand the control circuit is further configured to apply third controlsignals to the clutch control upon determining that the input and outputshafts are rotating at the same velocity, the third control signalscontrolling the clutch control to increase the pressure between theclutch plates at a predetermined pressure increase rate to maximumpressure which inhibits relative motion of the input and output shaftsuntil the torque transferred between the shafts exceeds a predeterminedlimit which is less than the predetermined maximum torque.
 6. The systemof claim 5, wherein the control circuit generates time valuesrepresentative to time periods when the third control signals areapplied to the clutch control, the input signals and output signals arenot representative of input and output rotational shaft speeds which aresubstantially equal, and the control circuit applies a fourth controlsignals to the clutch control when the time values exceed apredetermined limit, the clutch control disengaging the clutch plates inresponse to the fourth signal.
 7. A tractor comprising:a power sourcefor producing rotational motion; a power take-off (PTO) shaft forsupplying rotational motion to at least one piece of equipment; a clutchincluding an input shaft coupled to the power source and an output shaftcoupled to the PTO shaft, wherein the clutch includes a plurality ofclutch plates operable to translate through a distance prior to clutchengagement and, upon clutch plate engagement, transmitting torquebetween the input and output shafts; a clutch control coupled to theclutch and configured to translate the clutch plates through thedistance to engage the clutch plates in response to a first controlsignal; an output transducer disposed to generated an output signalrepresentative to the rotational speed of the output shaft; and acontrol circuit coupled to the clutch control, and the outputtransducer, and being configured to monitor the output signal and applythe first control signal to the clutch control until the output signalis representative of a rotational speed of the output shaft greater thanzero.
 8. The tractor of claim 7, further comprising a brake coupled tothe clutch control and disposed to inhibit rotation of the output shaftwhen the clutch is disengaged and when the first control signal isapplied to the clutch control.
 9. The tractor of claim 7, furthercomprising:a brake control coupled to the control circuit; and a brakecoupled to the brake control, the brake being disposed to inhibitrotation of the output shaft when the clutch is disengaged and when thefirst control signal is applied to the clutch control.
 10. The tractorof claim 7, wherein the torque transmitted by the clutch between theinput and output shafts is dependent upon the pressure between theclutch plates and the clutch control is further configured to force theclutch plates together at a pressure defined by second control signalsapplied to the clutch control, the system further comprising an inputtransducer disposed to generate an input signal representative of therotational speed of the input shaft, the input transducer being coupledto the control circuit and the control circuit being further configuredto monitor the input signal and, upon monitoring an output shaft speedgreater than zero, applying second control signals to the clutch controlto increase the pressure between the clutch plates at a pressurizationrate which transmits torque from the input to the output shaft at levelswhich increase the rotational velocity of the output shaft at a rateless than a predetermined rate.
 11. A tractor comprising:a power sourcefor producing rotational motion; a power take-off (PTO) shaft forsupplying rotational motion to at least one piece of equipment; a clutchincluding an input shaft coupled to the power source and an output shaftcoupled to the PTO shaft, wherein the clutch includes a plurality ofclutch plates operable to translate through a distance prior to clutchengagement and, upon clutch engagement, transmitting torque betweeninput and output shafts; a clutch control coupled to the clutch andconfigured to translate the clutch plates through the distance to engagethe clutch plates in response to a first control signal; an outputtransducer disposed to generate an output signal representative or therotational speed of the output shaft; and a control circuit coupled tothe clutch control, and the output transducer, and the being configuredto monitor the output signal and apply the first control signal to theclutch control until the output signal is representative of a rotationalspeed of the output shaft greater than zero, wherein the torquetransmitted by the clutch between the input and output shafts isdependent upon the pressure between the clutch plates and the clutchcontrol is further configured to force the clutch plates together at apressure defined by second control signals applied to the clutchcontrol, the system further comprising an input transducer disposed togenerated an input signal representative of the rotational speed of theinput shaft, the input transducer being coupled to the control circuitand the control circuit being further configured to monitor the inputsignal and, upon monitoring an output shaft speed greater than zero,applying second control signals to the clutch control to increase thepressure between the clutch plates at a pressurization rate whichtransmits torque from the input to the output shaft at levels whichincrease the rotational velocity of the output shaft at a rate less thana predetermined rate, and wherein the PTO shaft will fail at apredetermined maximum torque and the control circuit is furtherconfigured to apply third control signals to the clutch control upondetermining that the input and output shafts are rotating at the samevelocity, the third control signals controlling the clutch control toincrease the pressure between the clutch plates at a predeterminedpressure increase rate to a maximum pressure which inhibits relativemotion of the input and output shafts until the torque transferredbetween the shafts exceeds a predetermined limit which is less than thepredetermined maximum torque.
 12. The tractor of claim 11, wherein thecontrol circuit generates time values representative of time periodswhen the third control signals are applied to the clutch control, theinput signals and output signals are not representative of input andoutput rotational shaft speeds which are substantially equal, and thecontrol circuit applies a fourth control signals to the clutch controlwhen the time values exceed a predetermined limit, the clutch controldisengaging the clutch plates in response to the fourth signal.
 13. Thetractor of claim 11, further comprising a source of pressurizedhydraulic fluid, the clutch being a hydraulic clutch wherein the platesare engageable at an engagement pressure related to the hydraulicpressure applied to the clutch, the clutch control including a hydraulicvalve for coupling the clutch to the source of pressurized hydraulicfluid, and the hydraulic valve being a proportional valve configured tocontrol the pressure of the fluid applied to the clutch from the source,wherein the pressure is dependent upon the first, second and thirdcontrol signals.
 14. The tractor of claim 13, further comprising:ahydraulically operated brake coupled to the source of pressurizedhydraulic fluid and disposed to inhibit rotation of the output shaftwhen the hydraulic clutch is disengaged.
 15. The tractor of claim 14,wherein the hydraulic valve couples the brake to the source ofpressurized hydraulic fluid.
 16. The tractor of claim 14, furthercomprising a hydraulic brake valve coupled between the source ofpressurized hydraulic fluid and the brake.
 17. The tractor of claim 13,wherein the control circuit includes a digital processor configured toproduce the first, second and third control signals which arepulse-width modulated signals having a predetermined frequency, and thepressure applied to the clutch is substantially proportional to thepulse-width of the modulated signals.
 18. A power take-off (PTO) clutchcontrol system for use in a tractor including a power take-off (PTO)shaft for supplying rotational motion to at least one piece of equipmentother than the tractor, and a multi-plate clutch including an inputshaft coupled to a power source and an output shaft coupled to the PTOshaft, wherein the clutch plates are operable to translate through adistance prior to clutch engagement and, upon clutch plate engagement,transmit torque between the input and output shafts, the systemcomprising:movement means for translating the clutch plates through thedistance to engage the clutch plates in response to a first controlsignal; transducer means for generating an output signal representativeof the rotational speed of the output shaft; and control means formonitoring the output signal and applying the first control signal tothe clutch control until the output signal is representative of arotational speed of the output shaft greater than zero.
 19. The systemof claim 18, further comprising a means coupled to the clutch controland disposed to inhibit rotation of the output shaft when the clutchplates are disengaged and when the first control signal is applied tothe movement means.
 20. The system of claim 18, wherein the torquetransmitted by the clutch plates between the input and output shafts isdependent upon the pressure between the clutch plates and the movementmeans forces the clutch plates together at a pressure defined by secondcontrol signals applied to the movement means, the system furthercomprising an input means for generating an input signal representativeof the rotational speed of the input shaft, the control circuitmonitoring the input signal and, upon monitoring an output shaft speedgreater than zero, applying second control signals to the movement meansto increase the pressure between the clutch plates at a pressurizationrate which transmits torque from the input to the output shaft at levelswhich increase the rotational velocity of the output shaft at a rateless than a predetermined rate.
 21. A power take-off (PTO) clutchcontrol system for use in a tractor including a power take-off (PTO)shaft for supplying rotational motion to at least one piece of equipmentother than the tractor, and a multi-plate clutch including an inputshaft coupled to a power source and an output shaft coupled to the PTOshaft, wherein the clutch plates are operable to translate through adistance prior to clutch engagement and, upon clutch plate engagement,transmit torque between the input and the output shafts, the systemcomprising:movement means for translating the clutch plates through thedistance to engage the clutch plates in response to a first controlsignal; transducer means for generating an output signal representativeof the rotational speed of the output shaft; and control means formonitoring the output signal and applying the first control signal tothe clutch control until the output signal is representative of arotational speed of the output shaft greater than zero; wherein thetorque transmitted by the clutch plates between the input and outputshafts is dependent on the pressure between the clutch plates and themovement means forces the clutch plates together at a pressure definedby second control signals applied to the movement means, the systemfurther comprising an input means for generating an input signalrepresentative of the rotational speed of the input shaft, the controlcircuit monitoring the input signal and, upon monitoring an output shaftspeed greater than zero, applying second control signals to the movementmeans to increase the pressure between the clutch plates at apressurization rate which transmits torque from the input to the outputshaft at levels which increase the rotational velocity of the outputshaft at a rate less than a predetermined rate, and wherein the PTOshaft will fail at a predetermined maximum torque and the control meansapplies third control signals to the movement means upon determiningthat the input and output shafts are rotating at the same velocity, thethird control signals controlling the movement means to increase thepressure between the clutch plates at a predetermined pressure increaserate to a maximum pressure which inhibits relative motion of the inputand output shafts until the torque transferred between the shaftsexceeds a predetermined limit which is less than the predeterminedmaximum torque.
 22. The system of claim 21, wherein the control meansgenerates time values representative of time periods when the thirdcontrol signals are applied to the movement means, the input signals andoutput signals are not representative of input and output rotationalshaft speeds which are substantially equal, and the control meansapplies a fourth control signals to the movement means when the timevalues exceed a predetermined limit, the movement means disengaging theclutch plates in response to the fourth signal.